This invention relates to laminated safety glass and more particularly to a polymeric laminate which includes a layer of plasticized polyvinyl butyral (PVB) for use in such structures.
The present most widely used vehicle windshield is a three ply laminate comprising an energy absorbing plastic sheet sandwiched between two glass sheets Such windshields can absorb a blow from an occupant's head without shattering and in doing so dissipate significant impact energy as the windshield undergoes essentially plastic deformation.
With such a three ply structure the occupant can receive facial lacerations from sharp edges of broken glass of the inner glass sheet of the windshield. To reduce this the inside glass surface has been coated or covered with a protective plastic layer intended to prevent an occupant's skin from contacting the glass and further increase the penetration resistance of the conventional three ply laminate. Such protective layer usually comprises two or more plies and is known as an anti-lacerative shield (ALS). Alternatively, to address this occupant laceration problem and improve vehicle fuel efficiency by reducing weight, it has been proposed to reduce the conventional three ply structure to a single glass layer having a layer of energy absorbing material and a protective covering on its inboard side. Such a composite structure is known as a bilayer windshield and is especially suitable for ease of fabrication into relatively complex shapes.
As typically represented by U.S. Pat. No.4,584,229, for bilayer and ALS structures the prior art proposes spraying special adhesive polyurethanes (PU) onto the glass layer over which is then deposited a different, wear-resistant inboard polyurethane layer. However, sprayed liquid adhesive coatings are not usually encountered in commercial glass laminating systems which have long been based on polyvinyl butyral (PVB) with which laminators are quite familiar in handling and use. Such polyvinyl butyral is more particularly described as partial polyvinyl butyral insofar as containing about 17 to 25 weight % residual hydroxyl groups which promote adhesion to the glass. Moreover, plasticized polyvinyl butyral or partial polyvinyl butyral (interchangeably referred to as "PVB") as an energy absorbing layer (sometimes referred to as "PVB layer") has performance advantages over polyurethane. For example, the impact tear resistance of a PVB layer is greater than polyurethane over most of the operating temperature range encountered in vehicle window use. At thicknesses providing the expected degree of safety in use, the component cost of an all-polyurethane system is about twice that of one containing a PVB layer. Optical clarity can and has been designed into plasticized PVB sheet supplied by sheet manufacturers to glass laminators which may be more difficult to achieve when being developed by depositing liquid polyurethane susceptible to uneven gravity flow on a contoured automotive window panel.
In copending application Ser. No. 279,742, Filed Dec. 5, 1988 and assigned to the assignee of this application, a polymeric laminate containing a PVB layer for laminating to a glass sheet in bilayer and ALS applications is disclosed. As known, such a soft, vulnerable PVB layer must be covered with an additional inboard layer for various performance reasons which include: (i) prevention of plasticizer loss since if this occurs impact absorption is dissipated; and (ii) protection from abrasion, staining, cleaning solvents and various forms of damaging environmental exposure, one or more of which are likely to occur in use. Obviously, the bond between the PVB and covering layers is important and desirably strong to preserve the integrity of the laminated system in use. Before laminating to a PVB layer some plastic covering layers require initial surface treatment or the presence of a separate tie layer to promote adhesion. Nevertheless, over time a reduction in bond strength has been noted with prior art systems of this latter type. It would be desirable in a polymeric laminate for bilayer and ALS applications containing a PVB layer to provide a strong bond between it and a protective covering layer with which it is in face-to-face contact.